My vote goes for Mozilla Thunderbird - it's free, it's very user-friendly, it has everything inbuilt into the programming that enables it to work with almost every file type and attachment type - and it works without bugs, or crashes, or the requirement for "urgent" daily or weekly updates or upgrades.
I originally started with Outlook Express when I originally started on computers with Windows 98 (in 1998) - and I moved over to Thunderbird about 5 or 6 yrs ago, and have never regretted it.
Use it with Mailwasher to inspect and "wash" your mail before you download it, and you also reduce your chances of receiving viruses, or nasty and unwanted garbage in scam emails, to virtually nil.